Radio receiver antenna system



July s, 1969 w E GUSTAFSON ET A. 3,454,949

RADIO RECEIVER ANTENNA SYSTEM Filed Nov. 2z, 196e sheet of 2 v I? 2M 1 m) w/LBUR E. GUsrAFsO/ WALTER M. CHASE BY JAMES L. LIEVE/vs JA MES D. CAMPBELL Juy 8, 1969 WI E, GUSTAFSQN ET Al. 3,454,949

RADIO RECEIVER ANTENNA SYSTEM Sheet Filed Nov. 22, 1966 FR EQUENCY (Mc/S) wvl RECEIVER STEPPING MOTOR NOISE GENERATOR REVERSING INVENTOR` W/LBUR E. GUSTAFSO/V WALTER M. CHASE BY JAMES L.

[./EVENS JMES. CAMPBELL @Agro-EN WH United States Patent O 3,454,949 RADIO RECEIVER ANTENNA SYSTEM Wilbur E. Gustafson, Walter M. Chase, James L. Lievens,

and `lames D. Campbell, San Diego, Calif., assignors to the United States of America as represented by the Secretary of the Navy Filed Nov. 22, 1966, Ser. No. 596,340 Int. Cl. H01q 9/44 U.S. Cl. 343-745 4 Claims The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

This invention relates to antenna systems and is particularly directed to means for decreasing the coupling between closely spaced antennas. This -invention provides means for vcontrolling the effectiveness of receiving antennas which must be mounted closely adjacent to transmitting antennas and yet must operate in broad over-lapping frequency bands.

Aboard a ship with limited available deck space, electromagnetic fields, caused by the adjacent super structures, create a difficult design problem 4for high frequency radio receiving systems. There is insufficient room for the erection of the required number of both transmitting and receiving antennas, particularly those of the broadband type, with proper spacing between antennas. Antennas operable over broad frequency spectrums, rather than narrowly tuned antennas are required to reduce the total number of antennas, whereupon multicouplers are employed to couple the receivers and transmitters to the antennas.

Because of the complexity of shipboard radio gear it is no longer practicable to mount clothes-line antennas between masts. Instead, the small or miniature antennas of the whip-type are required. It is not necessary for a receiving antenna to have an efficiency of more than 70%, unlike the transmitting antenna. This is because of the relatively high atmospheric or local shipboard noise which overrides the receiver noise when an efficient antenna is used with a receiver of good noise factor. If the noise picked up by a receiving antenna is significantly greater than the reeciver noise, further increase in the efficiency of the receiving antenna will not improve the overall signal-to-noise ratio. Atmospheric noise expressed in decibels above thermal agitation noise can be expected to exceed 35 db at 2 megacycles per second, 30 db at 1() mc./s. and 20 db at 20 mc./s.

This invention takes advantage of the fact that the gain or efficiency of a receiving antenna can be considerably reduced without sacrificing signal receivability thus making it feasible to use smaller, less eicient antennas; and thus to reduce antenna interaction and conserve deck space.

The object of this invention is to provide an improved antenna system where a wide spectrum of frequencies must be received from a plurality of closely spaced antennas.

A more specic object of this invention is to provide an arrangement of miniature or whip-type antennas which will meet the stated operating requirements.

A further complicating factor in the design of shipboard arrays resides in the fact that tuned antennas less than one quarter wavelength long increases in eiciency very rapidly as the opearting frequency approaches the quarterwave condition. Accordingly, a further object of this invention is to provide means for controlling the efficiency-frequency characteristic of each antenna so that the signal-tonoise ratio remains within predictable ranges.

The objects of this invention are attained by a tower upon which is mounted a whip-type antenna or a plurality ICC of whip-type antennas, the length of which is short compared to the wavelength of the highest operating frequency. A resonant circuit comprising a coil and tuning condenser tuneable to resonance over the desired operat- -ing band is coupled between the antenna and the utilization circuit. One end of the resonant circuit is grounded to the upper end of the tower and the other end of the resonant circuit is coupled through a coupling condenser to the whip. Finally, an adjustable shunting condenser is connected between the antenna terminal and the towerground and is ganged with the tuning condenser in such a Way that the shunting condenser increases in capacity as the resonant frequency of said resonant circuit increases to attenuate the available signal and hence reduce the efficiency of the antenna system at the higher frequencies. This technique attens somewhat the efficiency-frequency characteristic of the whip.

Other objects and features of this invention will become apparent to those skilled in the art by referring to the spec-ic embodiments suggested in the following speciiication and shown in the accompanying drawings in which:

FIG. 1 is a schematic circuit diagram of one yantenna coupling system of this invention;

FIG. 2 is a perspective view of a tower according to this invention for supporting a large number of whip antennas;

FIG. 3 is a schematic diagram with switching circuits of an operating antenna tuneable in several bands;

FIG. 4 is a frequency versusl attenuation diagram showing the efiiciency of an antenna throughout a broad band according to this invention.

According to the embodiment of this invention as shown in FIG. 2 a plurality of minaiture antennas 1 are mounted on a tower structure 2. The tower can be constructed of angle irons and cross braced for structural rigidity and can be preferably, welded to the deck structure to stand free without guy wires. A certain of vertical wires spaced six inches apart surround the tower are welded to horizontal members of the tower to insure a continuous electrical structure of uniform dimensions on a ground plane such as the steel decking 2A. The tower according to one successful embodiment is 3 feet square and 25 feet high The miniature antenna chose was of the whip-type and for the particular ship installation considered the selected whip was 5 feet long. The -purpose of the tower is to obtain an independent system free of the irregularities of the super-structure of the ship. The advantage of the tower of FIG. 2 over the super-structure of the ship is that it is of uniform electrical characteristics which has a predictable vertical plane pattern and which makes it easy to predict the impedance matching of the tuner. A mechanical advantage of the tower is that it provides freedom of location so that azimuthal patterns may be improved in some instances, and locations more distant from transmitting antennas may be selected. According to this invention each whip antenna is separately tuned and the tuning and coupling circuit is placed in and connected to a metal container, at the base of the whip, which in turn is bolted and electrically connected to the upper portions of the tower. This results in a large enhancement of the radiation resistance of the otherwise relatively small antenna by forcing the signal current to flow in the vertical structure of the tower which has a heighth greater than that of the antenna. The vertical structure of the tower may be considered the antenna and the short tuned whip a means for coupling the antenna to the receiver.

The generalities of this invention may best be understood by rst referring to a specific embodiment. The circuit diagram of FIG. l shows the antenna 1 coupled to the parallel resonant circuit 11 comprising tuning coi1 12 and tuning condenser 13. The utilization circuit consistlng of a radio receiver 1f) is, in this case, tapped to a lower turn on the coil 12. It is intended that the resonant circuit 11 be tuneable throughout the expected frequency range to be received. Where this range covers four octaves, which in one military application consists of the range from two megacycles to eight megacycles, it was found expedient to switch several coils 12 successively into circuit to tune in different portions of this vbroad band.

The next problem solved by the circuits of FIG. 1 consists in minimizing the rate of increase of coupling efiiciency, as frequency of operation increases, between the antenna and receiver which is undesirable where high local noise levels may be encountered. To this end the coupling condenser 14 was connected between the antenna and the upper end of the tuning circuit 11, the other end of the tuning being grounded. Finally, the variable shunting condenser 15 is connected between the antenna 1 and ground, ground here being the upper end of the tower 2 to which the remaining elements of the coupler are connected. The purpose of the shunting condenser 15 is to bypass to ground more of the available antenna voltage as the frequency increases to thereby flatten the frequency-versus-efficiency characteristic of the system, and attain a more highly selective tuner circuit. Stated differently, since the capacity of the shunting condenser 15 is in parallel with the antenna-to-ground capacity, Ca, and s1nce the radiation resistance, Ra, changes but little, the Q of the antenna circuit can be controlled and hence the efficiency of the antenna system can be controlled. The condenser 15 is ganged with the tuning condenser 13 so that one increases while the other decreases. This means that, for example, when tuning condenser 13 is at a minimum for resonance at the highest frequency, the condenser 15 is at a maximum for maximum shunting and for reduced antenna gain. For a tuning range from about three to five megacycles, the tuning coil 12 should be about 14 ith. while the tuning condenser 13 should be smoothly variable from about 14 to 154 picofarads. From the known radiation resistance at the base of the whip antenna mounted on the tower it was calculated, and proven empirically, that the shunting condenser 15 should have a value variable between 6 and 75 picofarads. Linkage 16 such as a rotary tuning shaft connects Condensers 13 and 15.

The two-to-eight megacycle band can be divided into three parts as shown in the circuits of FIG. 3. Condensers 13 and 15 of FIG. 3 serve the same function as in FIG. l, and tuning coils 12a, 12b and 12C serve the same function as coil 12 in FIG. l. The several decks of the wafer switch 16 are keyed to the shaft 17A of the stepping motor 17 to move the contacts of the wafers into their several rotary positions. The bands are selected remotely by the operator. The precise tuning of the condenser 13-15 by the reversible synchro motor 18 also controlled remotely -by the operator A noise generator 19 may, if desired, be connected to one of the switch contacts to inject noise which can be employed for tuning the antenna for maximum transfer of energy to the receiver. The tuning ranges of the three coils 12a, 12b and 12e are, respectively, from 2 to 3.5, 3.5 to 5, and 5 to 9 megacycles per second. In the tuner shown the two variable capacitors 13 and 15 are used on the second and third bands only, a main tuning capacitor of 154 pf. and an auxiliary tuning capacitor Of 75 pf. Band 1 does not have the auxiliary shunting capacitor in order to obtain maximum permissible eficiency in the 2 megacycle region.

The operating characteristics shown in FIG. 4 was obtained by plotting the signal voltage at the loutput terminals of the tuner of FIG. 3 for all frequencies from 2 to 8 mc. A reference signal voltage is first obtained by measuring the signal at the end of an ideal antenna one quarter wavelength long at the operating frequency. Preferably, the input to the receiver is measured in decibels below this reference. As shown in FIG. 4 the characteristic is divided into three sections corresponding to the three tuning coils 12a, 12b and 12e as they are connected in. The shaded areas of FIG. 4 are obtained by taking readings for various resonant frequency spacings and various physical arrangements of whips on the tower. The heavy line of FIG. 4 is for a frequency separation of 5 percent or greater. The efficiency of the system from 2 to 8 megacycles is relatively constant.

It is apparent, then, that a large number of miniature antennas can be mounted on an electrically stable tower and operated over a broadband of frequencies without interference with the efiiciency controlled to that level to insure receivability limited only by external noise. The selectivity of the tuner, with controlled eciency, is much greater-.than if maximum efiiciency were the goal.

Many modifications may be made in the parameters of the system of this invention without departing from the scope of the appended claims.

What is claimed is:

1. In an antenna system for operating over a relatively broad band of frequencies;

a whip-type antenna, the length of the antenna being less than a quarter wave at the highest frequency of said band,

a resonant circuit comprising a parallel coil and condenser tunable to resonance over a predetermined portion of said band, a utilization circuit coupled to the resonant circuit, one end of said resonant circuit being connected to ground,

a coupling condenser connected between the other end of said resonant circuit and said antenna, and

an adjustable shunting condenser connected between said antenna and ground and ganged with said tuning condenser, said shunting condenser being increased in capacity as the resonant frequency of said resonant circuit increases to reduce the efficiency of the antenna system at the higher frequencies7 in a controlled manner.

2. The antenna system defined in claim 1 further comprismg;

a supporting tower for said whip-type antenna, said tower being a free standing metal structure sufficiently tall to support said antenna above ground a distance equal to approximately one quarter of a wavelength at said highest frequency so that the high frequency energy flowing in said tower is effectively coupled in the antenna circuit.

3. The tower defined in claim 2 further comprising;

relatively closely spaced vertical wires parallel to the vertical structural members of said tower and electrically connected to cross members to give said tower a reliable uniform dimension and stable ground plane for said high frequency energy.

4. In combination in an antenna system;

an antenna which is short in terms of the operating wave length,

a tunable resonant circuit coupled between said antenna and a utilization means; and

an adjustable condenser coupled in shunt across said resonant circuit, the adjustable means of said condenser being interlocked with the tuning means of said resonant circuit so that the capacity of said condenser increases as the resonant frequency of said resonant circuit increases for decreasing the effectiveness of the resonant coupling circuit as the operating frequency increases.

No references cited.

HERMAN KARL SAALBCH, Primary Examiner.

M. NUSSBAUM, Assistant Examiner.

U.S. Cl. X.R. 343--848, 861, 874 

1.IN AN ANTENNA SYSTEM FOR OPERATING OVER A RELATIVELY BROAD BAND OF FREQUENCIES; A WHIP-TYPE ANTENNA, THE LENGHT OF THE ANTENNA BEING LESS THAN A QUARTER WAVE AT THE HIGHEST FREQUENCY OF SAID BAND, A RESONANT CIRCUIT COMPRISING A PARALLEL COIL AND CON-NDENSER TUNABLE TO RESONANCE OVER A PREDETERMINED PORTION OF SAID BAND, A UTILIZATION CIRCUIT COUPLED TO THE RESONANT CIRCUIT, ONE END OF SAID RESONANT CIRCUIT BEING CONNECTED TO GROUND, A COUPLING CONDENSER CONNECTED BETWEEN THE OTHER END OF SAID RESONANT CIRCUIT AND SAID ANTENNA, AND AN ADJUSTABLE SHUNTING CONDENSER CONNECTED BETWEEN SAID ANTENNA AND GROUND AND GANGED WITH SAID TUNING CONDENSER, SAID SHUNTING CONDENSER BEING INCREASED IN CAPACITY AS THE RESONANT FREQUENCY OF SAID RESONANTT CIRCUIT INCREASES TO REDUCE THE EFFICIENCY OF THE ANTENNA SYSTEM AT THE HIGHER FREQUENCIES, IN A CONTROLLEDD MANNER. 